a semianalytical p / z technique for the analysis of abnormally pressured gas reservoirs
DESCRIPTION
SPE 71514. A Semianalytical p / z Technique for the Analysis of Abnormally Pressured Gas Reservoirs. Ronald Gunawan Gan, VICO Indonesia and T. A. Blasingame, Texas A&M University. Objective. To present a new technique that can be used to : Calculate gas-in-place for an abnor- - PowerPoint PPT PresentationTRANSCRIPT
A Semianalytical p/z Technique for the Analysis of
Abnormally Pressured Gas Reservoirs
Ronald Gunawan Gan,VICO Indonesia
andT. A. Blasingame,
Texas A&M University
SPE 71514
ObjectiveTo present a new technique that can beused to : Calculate gas-in-place for an abnor-
mally pressured gas reservoir using only average reservoir pressure and cumulative production data.
Calculate pore volume compressibi-lity as a function of reservoir pressure.
Presentation Outline Introduction
Overview of Existing Methods
New Method Field Examples Conclusions
Introduction p/z schematic for a normally-pressured volumetric gas reservoir
G
p/z
Gp
GG
zp
zp p
i
i 1
Introduction p/z schematic for an abnormally-pressured gas reservoir
p/z
Gp G
GG
zppp
zp p
i
ii 1)(1
Gapp
Introduction
Reasons for the non-linear p/z behavior:
Rock and water compressibility effects — "rock collapse theory" (Hawkins, 1969)
Shale water influx (Bourgoyne, 1989)
Existing Methods Methods based on presumed knowledge of system compressibility:
Hammerlindl (Constant Compressibility), 1971
Ramagost & Farshad (Constant Comp.), 1981
Yale et al. (Variable Compressibility), 1993
GG
zp
SccSp
zp p
i
i
w
fww 1)1(
)(1
Methods based on presumed knowledge of system compressibility (continued)
Fetkovich, Reese, and Whitson - 1991 - Derived General Material Balance Eq. - Define cumulative effective compressibility,
wi
ftwftwwie S
pcpcMpcpcSpc
1
)]()([)()()(
- ce represents the cumulative change in hydrocarbon PV caused by compressi- bility effects (and water influx).
Methods which do not require a prior knowledge of system compressibility
Roach - 1981 - very sensitive to initial pressure.- method sometimes doesn’t exhibit a negative intercept (which is not possible).
Bernard - 1985 - using Least Squares approach. - very sensitive to data scatter.
Ambastha - 1991: Type Curve Approach - non-uniqueness problems.
New Method
Develops 2 new plotting functions:
1. )/)/(/(versus)( iiie zpzpppc
2. /GGzpzp pii versus)/)/(/(
Requires production data only (p and Gp)
Satisfies both "rock collapse" and "shale water influx" theories
New Method Uses general material balance equation (proposed by Fetkovich, et al.)
GG
zpppc
zp p
i
iie 1)(1
Rearranging, we obtain
GG
zpzpppc pii
ie 1//1)(
New Method Calculate the ce(pi-p) function for each p/z versus Gp trend
ce(pi-p) = ???
ce(pi-p) = ???
Gp
p/z
G Gapp
New Method For early time data (1st straight line) :
GG
GG
ppc app
zp
zp
app
zp
zpie
i
i
i
i
)/(1
)/(11)(
For late time data (2nd straight line) :
GG
ppc pA
zpzpie
iiA
111)()/(
)/(
where: A is the inflection point
New Method
Plot of log ce(pi-p) versus (p/z)/(pi/zi):
(p/z)/(pi/zi)
h
log
c e(p
i-p) G/Gapp=0.7
G/Gapp=0.6
G/Gapp=0.8
inflection point
Plot of log ce(pi-p) versus (p/z)/(pi/zi) :
(p/z)/(pi/zi)
h
log
c e(p
i-p)
inflection point
New Method
New Method /GGzpzp pii versus)/)/(/(
Gp/G
h
(p/z
)/(p i
/zi)
0 1
1
Infl. Point: GpA/G, (p/z)A /( pi /zi )
GG
GG1
/zpp/z p
appii
GG
GGzpzp
/zpp/z p
pAii
A
ii )/1)(/()/(
New Method /GGzpzp pii versus)/)/(/(
Gp/G
h
(p/z
)/(p i
/zi)
0 1
1
G/Gapp=1G/Gapp= 0.8
G/Gapp=0.6
Inflection point
New Method /GGzpzp pii versus)/)/(/(
Gp/G
h
(p/z
)/(p i
/zi)
0 1
1Inflection point
G/Gapp=0.8
New Method
/GGzpzp pii versus)/)/(/( Dynamic Type Curve Matching. Automatic Matching using SOLVER m(Excel function for non-linear regression).
New Method
Data required for analysis: Fluid property data Initial Reservoir p and T p and Gp data
New Method
Computer program: Visual Basic Application in MS Excel
Easy to use - especially for analysis Only requires MS Excel
Data Analysis Sheet
Example 1: G is too low
Example 1: G is too high
Example 1: Correct G
Example 2: Long transition period
Example 3: Early time data
Example 4: Synthetic Dry Gas Case
Example 4: Backcalculated cf
Procedure to calculate cf vs. p from production data:
1. Get )( pce from type curve matching
3. Calculate cf (p):
jfnif pcppcn
jj
1)(
wi
ftwftwwie S
pcpcMpcpcSpc
1
)]()([)()()(
2. Use the following equation to calculate )( pc f :
Example 4: Backcalculated cf
Conclusions We have developed a straightforward approach for analyzing p/z versus Gp
behavior for abnormally pressured gas reservoirs — the approach considers that two straight-lines must be ob- served on the p/z plot. The proposed method determines gas-in-place without using system compressibility data. Only p, Gp, and fluid property data are required.
Conclusions (continued)
Our approach of using ce(pi-p) versus (p/z)/(pi /zi) and (p/z)/(pi /zi) versus Gp/G as dynamic type curve matching func- tions has been shown to work extreme- ly well. Using our new method, it is possible to calculate rock compressibility as a func- tion of pressure from p and Gp data
Conclusions (continued)
The "dynamic type curve matching technique" used for calculating gas-in-place from production data is more representative (and more stable) than the non-linear optimization method provided by SOLVER.
A Semianalytical p/z Technique for the Analysis of
Abnormally Pressured Gas Reservoirs
Ronald Gunawan Gan,VICO Indonesia
andT. A. Blasingame,
Texas A&M University
SPE 71514